A light emitting diode is the most popular kind of optoelectronics.
To make the LED light up, you need a power supply (any voltage) and a resistor.
The resistance required is as follows: R = (U_power - U_led) / I_max
Example: a LED with voltage drop of Uled=1.2V and a max current of I_max=0,020A (20mA) to be powered by a 5V DC source needs:
R = (5-1.2)/.02 = 190 ohm
You then connect a 190 ohm resistance in series with the LED to make it work.
Several other wiki discuss how to turn LEDs into "throwies"(little blinking LED devices that attach to outdoor structures via magnets):
- Cybords wiki: Throwie Talkie: A Talkie is a Throwie that has been hacked to blink graffiti messages in Morse code.
- TagRecord.com Wiki: Led throwies
- the mayowiki: Throwies
A few people use LEDs in an unusual way: as sensors.
5.0v - 3.3V voltage drop via red LED
The average red LED has a 1.7V voltage drop. This property can be (ab)used to power very small 3.3V IC's from a 5.0V power supply -- put the LED in series with the 5V line. There probably won't be enough power used to light the LED properly, but this does not matter for the voltage drop property.
LEDs used normally: to emit light:
LEDs used "in reverse": as light sensors: